If you're currently using iBooks, Stanza or Bluefire Reader to read non-DRM ePubs, you should be aware of a free iOS reader ePub reader Marvin (iTunes link, Marvin home page).

Marvin picks up where Stanza left off, with fully configurable margins, line and paragraph spacing, variable fonts and color schemes and many more tweakable settings. It also has a very clever 'Deep view' feature which automatically generates character summaries.

Marvin plays well with calibre - it can connect to calibre's server to download books from your library to your device. You can also email books to Marvin, or load from cloud storage like Dropbox.

I like Marvin so much that I wrote an Annotations plugin which parses highlights and annotations exported from Marvin, adding them to calibre.

The developer is very responsive, with an active MobileRead thread here. He has indicated that an iPhone version is in the works, and eventually an Android version.